Galileo Observations of Gaspra, Ida, and Dactyl: Implications for Meteoritics

Galileo opportunisitically encountered the S-type asteroids Gaspra and Ida, and discovered Ida's satellite Dactyl. Returned data include high-resolution multi-color images and regional infrared reflectance spectra. Preliminary analyses [1-4] are supplemented by special issues of Icarus (Jan. 1994 and in press). Derived information includes shape, photometry, spectrophotometry, surface morphology, and crater statistics. This is an enormous leap; asteroids were previously studied telescopically with little spatial resolution. Rendezvous missions (like NEAR, February 1996 launch) can obtain masses and high-resolution, uniform, global data (including x-ray and gamma-ray). Nevertheless, we have learned much about the relationships between asteroids and meteorites. Surface geology and body shapes only hint at interior structure. Despite a dumb-bell aspect from some directions, much of Gaspra's shape is bounded by planes suggesting a monolithic interior. Ida more clearly has two lobes; its interior is rubblized to an unknown degree. Recent modelling of asteroidal fragmentation processes, studies of double craters, and observations of other asteroids imply that many smaller asteroids may be multi-component rubble piles. Asteroidal regolith processes differ from those on the Moon. Cratering ejecta are widespread (e.g. by Ida's large, fresh crater Azzurra) and much ejecta escapes a small asteroid entirely. Downslope movement of regolith is evident on both Gaspra and Ida. Ida's regolith may be tens of meters deep; it must be relatively immature, grading downward into coarser material (megaregolith or rubble-pile structure). Craters show a "steep" production population for Gaspra (i.e. dominated by small impactors). Larger craters do not saturate, so Gaspra's surface was re-set comparatively recently or Gaspra may be made of metallic-strength material. Ida is (nearly) saturated by craters; it could have been struck by the same population that struck Gaspra and the post-mare Moon. Some of Ida's craters may be from the break-up of the Koronis parent body. Dactyl was probably created during that break-up; small satellites around other asteroids may be common. "Space weathering," which slightly modifies spectral reflectance with time, is marginally evident on Gaspra and prominent on Ida. Most surfaces have spectra that are slightly redder, more linear, and with shallower absorption bands than for regions associated with fresh craters. The unknown mechanism appears analogous to the well-known (but still not fully understood) lunar space-weathering process. The spectrum for Dactyl (which is expected to retain the least regolith of the three bodies) is even less weathered. The spectral trend from weathered-Ida, to fresh-Ida, to Dactyl extrapolates toward spectra of ordinary chondrites (OC). Thus Ida (and other members of the Koronis family) may be of OC composition. Some or all of the S-IV asteroids may, therefore, be the "missing main-belt parent bodies" for OC's. This possibility is bolstered by the unexpected finding [4] that Ida's bulk density is only ~2 1/2 g/cm^3. Compositions that are half metallic (e.g. pallasites or mesosiderites) are ruled out even for a rubble-pile structure. However, Gaspra has been known, since before the Galileo fly-by, to be too olivine-rich to be an OC parent body. Possibly it is roughly pallasitic in composition and structure. The Galileo observations, and other recent asteroid research, suggest a new -- though controversial, unproven -- paradigm: Meteorites (and their larger siblings, the Earth-approaching asteroids) are derived, in a fairly representative fashion, from main-belt asteroids, which include all common meteorite types, including OC's. The immediate (and intermediate) parent bodies are highly fractured, often with compound, rubble-pile-like structures, that represent a long history of cascading fragmentation. It is doubtful how much these modern parent bodies resemble the original bodies that condensed and accreted in the inner solar system. References: [1] Belton M. J. S. et al. (1992) Science, 257, 1647-1652. [2] Belton M. J. S. et al. (1994) Science, 265, 1543-1547. [3] Chapman C. R. et al. (1995) Nature, 374, 783-785. [4] Belton M. J. S. et al. (1995) Nature, 374, 785-788.